The requirements of silver halide photographic materials have increased sharply in recent years, and the development of photosensitive materials which exhibit higher sensitivities and excellent graininess is desirable. In order to improve the sensitivity and image quality of silver halide photographic materials it is necessary to improve the light absorption, quantum efficiency and developing properties of the silver halide emulsions.
Silver halide emulsions can be spectrally sensitized utilizing dyes. The absorption of as much dye as possible by the silver halide emulsion is advantageous from the point of view of light absorption. However, the utilization of these dyes serves to reduce the sensitivity of the silver halide emulsion. Therefore, an optimum spectral sensitization is achieved only using a smaller amount of dye than which would form a continuous mono-molecular layer on the surface of the grains in emulsions.
It has long been known that so-called internal latent image type emulsions, that is, emulsions which have chemical ripening specks with which a developable latent image can be formed by exposure to light (referred to below as sensitivity specks) within the grains, are effective as a means of overcoming this problem. For example, it has been indicated in U.S. Pat. No. 3,979,213 that the intrinsic densensitization on color sensitizing an internal latent image type silver halide emulsion is much less than that in the case of a silver halide emulsion of the same grain size which has been chemically sensitized only at the surface, and that as a result it is possible to achieve effective color sensitization using large amounts of sensitizing dye.
Much research has been done with these internal latent image type emulsions in connection with the formation of positive images by development using development baths which have a fogging action, as disclosed in JP-A-55-21067 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). However, these internal latent image type emulsions are generally such that the latent image is located within the grains at a depth of at least 100 nm from the surface, and thus they cannot be developed in surface development type development baths, such as color negative development baths.
On the other hand, emulsions in which the latent image is located in cavities which open toward the surface of the grains, whereby the desensitization due to the utilization of dyes of the type observed with the internal latent image type emulsions disclosed above is less likely to occur, and which can be developed in practical surface development type development baths, have been disclosed in U.S. Pat. No. 3,966,476.
Methods for the preparation of internal latent image type silve halide emulsions include those in which a fine grained non-chemically sensitized emulsion is mixed with large grains which have been chemically sensitized and the mixture is subjected to Ostwald ripening as disclosed in U.S. Pat. No. 3,206,313, and those in which silver ions and halide ions are added in such a way that they are alternately present in excess to grains which have been chemically sensitized, as disclosed in U.S. Pat. No. 3,917,485. It has been noted that it is possible to control the balance between the surface sensitivity and the internal sensitivity suitably by controlling the thickness of the shell when using these methods of preparation.
However, in general, the sensitivity specks are extremely small crystals of silver sulfide, gold silver sulfide, or silver which are epitaxially bonded to the silver halide crystals. Further, these specks are unstable and thus, changes at the internal latent image sites can occur on handling. This instability results not only in the formation of fog specks but also reduces their function as sensitivity specks. Consequently, it has not been possible to fully realize an increase in sensitivity which could be achieved by the provision of internal latent images.